Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 8355 S. Previdi, Ed.
Category: Informational Cisco Systems, Inc.
ISSN: 2070-1721 B. Decraene
Orange
R. Shakir
Google
March 2018
Resiliency Use Casesin Source Packet Routing in Networking (SPRING) Networks
Abstract
This document identifies and describes the requirements for a set of
use cases related to Segment Routing network resiliency on Source
Packet Routing in Networking (SPRING) networks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8355.
Filsfils, et al. Informational [Page 1]

RFC 8355 SPRING Resiliency Use Cases March 20181. Introduction
This document reviews various use cases for the protection of
services in a SPRING network. The terminology used hereafter is in
line with [RFC5286] and [RFC5714].
The resiliency use cases described in this document can be applied
not only to traffic that is forwarded according to the SPRING
architecture, but also to traffic that originally is forwarded using
other paradigms such as LDP signaling or pure IP traffic (IP-routed
traffic).
Three key alternatives are described: path protection, local
protection without operator management, and local protection with
operator management.
Path protection lets the ingress node be in charge of the failure
recovery, as discussed in Section 2.
The rest of the document focuses on approaches where protection is
performed by the node adjacent to the failed component, commonly
referred to as local protection techniques or fast-reroute techniques
[RFC5286] [RFC5714].
In Section 3, we discuss two different approaches providing unmanaged
local protection, namely link/node bypass protection and shortest-
path-based protection.
Section 4 illustrates a case allowing the operator to manage the
local protection behavior in order to accommodate specific policies.
In Section 5, we discuss the opportunity for the SPRING architecture
to provide loop-avoidance mechanisms such that transient forwarding
state inconsistencies during routing convergence do not lead into
traffic loss.
The purpose of this document is to illustrate the different use cases
and explain how an operator could combine them in the same network
(see Section 6). Solutions are not defined in this document.
Filsfils, et al. Informational [Page 3]

RFC 8355 SPRING Resiliency Use Cases March 2018
B------C------D------E
/| | \ / | \ / |\
/ | | \/ | \/ | \
A | | /\ | /\ | Z
\ | | / \ | / \ | /
\| |/ \|/ \|/
F------G------H------I
Figure 1: Reference Topology
We use Figure 1 as a reference topology throughout the document. The
following link metrics are applied:
o Links from/to A and Z are configured with a metric of 100.
o CH, GD, DI, and HE links are configured with a metric of 6.
o All other links are configured with a metric of 5.
Note: Link metrics are bidirectional; in other words, the same metric
value is configured at both sides of each link.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Path Protection
As a reminder, one of the major network operator requirements is path
disjointness capability. Network operators have deployed
infrastructures with topologies that allow paths to be computed in a
complete disjoint fashion where two paths wouldn't share any
component (link or router), hence allowing an optimal protection
strategy.
A first protection strategy consists of excluding any local repair
and instead uses end-to-end path protection where each SPRING path is
protected by a second disjoint SPRING path. In this case, local
protection is not used along the path.
For example, a pseudowire (PW) from A to Z can be "path protected" in
the direction A to Z in the following manner: the operator configures
two SPRING paths, T1 (primary) and T2 (backup), from A to Z.
Filsfils, et al. Informational [Page 4]

RFC 8355 SPRING Resiliency Use Cases March 2018
The two paths may be used:
o concurrently, where the ingress router sends the same traffic over
the primary and secondary path (this is usually known as 1+1
protection);
o concurrently, where the ingress router splits the traffic over the
primary and secondary path (this is usually known as Equal-Cost
Multipath (ECMP) or Unequal-Cost Multipath (UCMP)); or
o as a primary and backup path, where the secondary path is used
only when the primary failed (this is usually known as 1:1
protection).
T1 is established over path {AB, BC, CD, DE, EZ} as the primary path,
and T2 is established over path {AF, FG, GH, HI, IZ} as the backup
path. The two paths MUST be disjoint in their links, nodes, and
Shared Risk Link Groups (SRLGs) to satisfy the requirement of
disjointness.
In the case of primary/backup paths, when the primary path T1 is up,
the packets of the PW are sent on T1. When T1 fails, the packets of
the PW are sent on the backup path T2. When T1 comes back up, the
operator either allows for an automated reversion of the traffic onto
T1 or selects an operator-driven reversion. Typically, the
switchover from path T1 to path T2 is done in a fast-reroute fashion
(e.g., sub-50 milliseconds) but, depending on the service that needs
to be delivered, other restoration times may be used.
It is essential that any path, primary or backup, benefit from an
end-to-end liveness monitoring/verification. The method and
mechanisms that provide such a liveness check are outside the scope
of this document. An example is given by [RFC5880].
There are multiple options for a liveness check, e.g., path liveness,
where the path is monitored at the network level (either by the head-
end node or by a network controller/monitoring system). Another
possible approach consists of a service-based path monitored by the
service instance (verifying reachability of the endpoint). All these
options are given here as examples. While this document does express
the requirement for a liveness mechanism, it does not mandate, nor
define, any specific one.
Filsfils, et al. Informational [Page 5]

RFC 8355 SPRING Resiliency Use Cases March 2018
From a SPRING viewpoint, we would like to highlight the following
requirements:
o SPRING architecture MUST provide a way to compute paths that are
not protected by local repair techniques (as illustrated in the
example of paths T1 and T2).
o SPRING architecture MUST provide a way to instantiate pairs of
disjoint paths on a topology based on a protection strategy (link,
node, or SRLG protection) and allow the validation or
recomputation of these paths upon network events.
o The SPRING architecture MUST provide an end-to-end liveness check
of SPRING-based paths.
3. Management-Free Local Protection
This section describes two alternatives that provide local protection
without requiring operator management, namely bypass protection and
shortest-path-based protection.
For example, traffic from A to Z, transported over the shortest paths
provided by the SPRING architecture, benefits from management-free
local protection by having each node along the path automatically
precompute and preinstall a backup path for the destination Z. Upon
local detection of the failure, the traffic is repaired over the
backup path in sub-50 milliseconds. When the primary path comes back
up, the operator either allows for an automated reversion of the
traffic onto it or selects an operator-driven reversion.
The backup path computation SHOULD support the following
requirements:
o 100% link, node, and SRLG protection in any topology;
o automated computation by the IGP; and
o selection of the backup path such as to minimize the chance for
transient congestion and/or delay during the protection period, as
reflected by the IGP metric configuration in the network.
Filsfils, et al. Informational [Page 6]

RFC 8355 SPRING Resiliency Use Cases March 20183.1. Management-Free Bypass Protection
One way to provide local repair is to enforce a failover along the
shortest path around the failed component.
In case of link protection, the point of local repair will create a
repair path avoiding the protected link and merging back to the
primary path at the next hop.
In case of node protection, the repair path will avoid the protected
node and merge back to the primary path at the next-next hop.
In case of SRLG protection, the repair path will avoid members of the
same group and merge back to the primary path just after.
In our example, C protects destination Z against a failure of the CD
link by enforcing the traffic over the bypass {CH, HD}. The
resulting end-to-end path between A and Z, upon recovery from the
failure of CD, is depicted in Figure 2.
B * * *C------D * * *E
*| | * / * \ / |*
* | | */ * \/ | *
A | | /* * /\ | Z
\ | | / * * / \ | /
\| |/ **/ \|/
F------G------H------I
Figure 2: Bypass Protection around Link CD
When the primary path comes back up, the operator either allows for
an automated reversion of the traffic onto the primary path or
selects an operator-driven reversion.
Filsfils, et al. Informational [Page 7]

RFC 8355 SPRING Resiliency Use Cases March 20183.2. Management-Free Shortest-Path-Based Protection
An alternative protection strategy consists in management-free local
protection that is aimed at providing a repair for the destination
based on the shortest path to the destination.
In our example, C protects Z (which the traffic initially reaches via
CD) by enforcing the traffic over its shortest path to Z and
considering the failure of the protected component. The resulting
end-to-end path between A and Z, upon recovery from the failure of
CD, is depicted in Figure 3.
B * * *C------D------E
*| | * / | \ / |\
* | | */ | \/ | \
A | | /* | /\ | Z
\ | | / * | / \ | *
\| |/ *|/ \|*
F------G------H * * *I
Figure 3: Shortest Path Protection around Link CD
When the primary path comes back up, the operator either allows for
an automated reversion of the traffic onto the primary path or
selects an operator-driven reversion.
4. Managed Local Protection
There may be cases where a management-free repair does not fit the
policy of the operator. For example, in our illustration, the
operator may not want to have CD and CH used to protect each other
due to the bandwidth (BW) availability in each link that could not
suffice to absorb the other link traffic.
In this context, the protection mechanism MUST support the explicit
configuration of the backup path either under the form of high-level
constraints (end at the next hop, end at the next-next hop, minimize
this metric, avoid this SRLG, etc.) or under the form of an explicit
path. Upon local detection of the failure, the traffic is repaired
over the backup path in sub-50 milliseconds. When the primary path
comes back up, the operator either allows for an automated reversion
of the traffic onto it or selects an operator-driven reversion.
We discuss such aspects for both bypass and shortest-path-based
protection schemes.
Filsfils, et al. Informational [Page 8]

RFC 8355 SPRING Resiliency Use Cases March 20184.1. Managed Bypass Protection
Let us illustrate the case using our reference example. For the
demand from A to Z, the operator does not want to use the shortest
failover path to the next hop, {CH, HD}, but rather the path {CG, GH,
HD}, as illustrated in Figure 4.
B * * *C------D * * *E
*| * \ / * \ / |*
* | * \/ * \/ | *
A | * /\ * /\ | Z
\ | * / \ * / \ | /
\| */ \*/ \|/
F------G * * *H------I
Figure 4: Managed Bypass Protection
The computation of the repair path SHOULD be possible in an automated
fashion as well as statically expressed in the point of local repair.
4.2. Managed Shortest Path Protection
In the case of shortest path protection, the operator does not want
to use the shortest failover via link CH, but rather the traffic
should reach H via {CG, GH} due to constraints such as delay, BW, or
SRLG.
The resulting end-to-end path upon activation of the protection is
illustrated in Figure 5.
B * * *C------D------E
*| * \ / | \ / |\
* | * \/ | \/ | \
A | * /\ | /\ | Z
\ | * / \ | / \ | *
\| */ \|/ \|*
F------G * * *H * * *I
Figure 5: Managed Shortest Path Protection
The computation of the repair path SHOULD be possible in an automated
fashion as well as statically expressed in the point of local repair.
The computation of the repair path based on a specific constraint
SHOULD be possible on a per-destination prefix base.
Filsfils, et al. Informational [Page 9]

RFC 8355 SPRING Resiliency Use Cases March 20185. Loop Avoidance
It is part of routing protocols' behavior to have what are called
"transient routing inconsistencies". This is due to the routing
convergence that happens in each node at different times and during a
different lapse of time.
These inconsistencies may cause routing loops that last the time that
it takes for the node impacted by a network event to converge. These
loops are called "micro-loops".
Usually, in normal routing protocol operations, micro-loops do not
last long and are only noticed during the time it takes the network
to converge. However, with the emergence of fast-convergence and
fast-reroute technologies, micro-loops can be an issue in networks
where sub-50 millisecond convergence/reroute is required. Therefore,
the micro-loop problem needs to be addressed.
Networks may be affected by micro-loops during convergence depending
of their topologies. Detecting micro-loops can be done during
topology computation (e.g., Shortest Path First (SPF) computation),
and therefore techniques to avoid micro-loops may be applied. An
example of such technique is to compute a path free of micro-loops
that would be used during network convergence.
The SPRING architecture SHOULD provide solutions to prevent the
occurrence of micro-loops during convergence following a change in
the network state. Traditionally, the lack of packet steering
capability made it difficult to apply efficient solutions to micro-
loops. A SPRING-enabled router could take advantage of the increased
packet steering capabilities offered by SPRING in order to steer
packets in a way that packets do not enter such loops.
6. Coexistence of Multiple Resilience Techniques in the Same Infrastructure
The operator may want to support several very different services on
the same packet-switching infrastructure. As a result, the SPRING
architecture SHOULD allow for the coexistence of the different use
cases listed in this document, in the same network.
Filsfils, et al. Informational [Page 10]

RFC 8355 SPRING Resiliency Use Cases March 2018
Let us illustrate this with the following example:
o Flow F1 is supported over path {C, CD, E}
o Flow F2 is supported over path {C, CD, I}
o Flow F3 is supported over path {C, CD, Z}
o Flow F4 is supported over path {C, CD, Z}
It should be possible for the operator to configure the network to
achieve path protection for F1, management-free shortest path local
protection for F2, managed protection over path {CG, GH, Z} for F3,
and management-free bypass protection for F4.
7. Security Considerations
This document describes requirements for the SPRING architecture to
provide resiliency in SPRING networks. As such, it does not
introduce any new security considerations beyond those discussed in
[RFC7855].
8. IANA Considerations
This document has no IANA actions.
9. Manageability Considerations
This document provides use cases. Solutions aimed at supporting
these use cases should provide the necessary mechanisms in order to
allow for manageability as described in [RFC7855].
Manageability concerns the computation, installation, and
troubleshooting of the repair path. Also, necessary mechanisms
SHOULD be provided in order for the operator to control when a repair
path is computed, how it has been computed, and if it's installed and
used.
Filsfils, et al. Informational [Page 11]